Hydraulic devices as mentioned above include a hydraulic pump which rotates a pair of gears by an appropriate drive motor and pressurizes a working liquid by the rotational motions of the gears and discharges the pressurized working liquid, and a hydraulic motor which rotates gears by introducing a previously pressurized working liquid therein and uses rotational forces of rotating shafts of the gears as a power.
Such a hydraulic device generally has a configuration in which a pair of gears meshing with each other are contained in a housing and rotating shafts extended outward from both end surfaces of each gear are rotatably supported by bearing members which are contained in the same housing and disposed on both sides of each gear.
Conventionally, gears of various shapes have been used as the pair of gears and some hydraulic devices use helical gears as the pair of gears. Helical gears have a characteristic that, because of having a structure in which their teeth are oblique, gear tooth contact is spread and therefore noise is small, whereas they have a characteristic that, in a case where they are used as a hydraulic device, an axial force (thrust force) is generated by meshing of their teeth and further a thrust force is similarly generated by the fact that their tooth surfaces receive a pressure of the working liquid.
These thrust forces periodically vary due to rotations of the gears and such periodic variation causes a problem that noise is generated by vibration of the gears and the bearing members, or a problem that a gap is formed between the end surfaces of the gears and the end surfaces of the bearing members by the vibration and leakage from the high-pressure side to the low-pressure side through the gap is caused.
Accordingly, for solving these problems, there has been suggested a hydraulic device (specifically, a gear pump) configured to inhibit displacement of the gears in their axial directions by causing a force in the opposite direction (drag) greater than the above-described thrust forces to act on the rotating shafts (see the U.S. Pat. No. 6,887,055 (PTL 1)). A configuration of the gear pump described in the PTL 1 is shown in FIG. 17.
As shown in FIG. 17, a gear pump 100 has a body 101 having a hydraulic chamber 101a formed therein, and a pair of helical gears 115, 120 inserted in the hydraulic chamber 101a with their tooth portions meshing with each other. As for the pair of gears 115, 120, the gear 115 is a driving gear and the gear 120 is a driven gear, and their rotating shafts 116, 121 are rotatably supported by bushes 110a, 110b, 110c and 110d which are similarly inserted in the hydraulic chamber 101a. 
Further, a front cover 102 is liquid-tightly fixed to the front end surface of the body 101 by a seal, while an intermediate plate 106 is similarly liquid-tightly fixed to the rear end surface of the body 101 by a seal and a rear cover 104 is similarly liquid-tightly fixed to the rear end surface of the intermediate plate 106 by a seal. The body 101, the front cover 102, the intermediate plate 106 and the rear cover 104 together form a housing within which the hydraulic chamber 101a is sealed. It is noted that the rotating shaft 116, which is inserted through a through hole 102a of the front cover 102 and extended outward, is sealed by a not-shown seal between the outer peripheral surface of the rotating shaft 116 and the inner peripheral surface of the through hole 102a. 
The hydraulic chamber 101a is divided in two, a high-pressure side and a low-pressure side, at a meshing portion of the pair of gears 115, 120, and when the driving gear 115 is driven and rotated by an appropriate driving source and the pair of gears 115, 120 thereby rotate, a working liquid is introduced into the low pressure side through a not-shown intake port and the introduced working liquid is led to the high pressure side while being pressurized by an action of the pair of gears 115, 120, and the high-pressure working liquid is discharged through a not-shown discharge port.
Further, the intermediate plate 106 has through holes 106a, 106b bored therethrough at portions corresponding to the rotating shafts 116, 121, respectively, and pistons 108, 109 are inserted through the through holes 106a, 106b, respectively. Further, a concave hydraulic chamber 104a corresponding to a region including the through holes 106a, 106b is formed in the surface being in contact with the intermediate plate 106 (front surface) of the rear cover 104, and the working liquid in the high-pressure side is to be supplied into the hydraulic chamber 104a through an appropriate flow path. Furthermore, the working liquid in the high-pressure side is to be supplied into between the front surface of the intermediate plate 106 and the rear surfaces of the bushes 110a, 110c through an appropriate flow path.
According to the gear pump 100 having the above-described configuration, during the operation of the gear pump 100, the working liquid in the high-pressure side is supplied into the hydraulic chamber 104a of the rear cover 104, the pistons 108, 109 are pressed forward by the high-pressure working liquid, and the gears 115, 120 are pressed forward by the pistons 108, 109 via the rotating shafts 116, 121, and simultaneously the bushes 110a, 110c are pressed forward by the high-pressure working liquid supplied into between the front surface of the intermediate plate 106 and the rear surfaces of the bushes 110a, 110c. Due to these actions, the bushes 110a, 110c, the gears 115, 120 and the bushes 110b, 110d are integrally pressed forward and the bushes 110b, 110d are pressed onto the rear end surface of the front cover 102.
It is noted that the pressing force for integrally pressing a structure comprising the bushes 110a, 110b, the gears 115, 120 and the bushes 110b, 110d forward is set to be greater than the thrust forces generated by the rotations of the gears 115, 120. Further, the pistons 108, 109 have their respective pressure receiving areas (cross-sectional areas) which are respectively determined in accordance with the thrust forces acting on the driving gear 115 and the driven gear 120, and the cross-sectional area of the piston 108 is larger than that of the piston 109.
As described above, in a hydraulic device using helical gears, the thrust forces generated by rotations of the helical gears causes vibration and noise and causes leakage from the high pressure side to the low pressure side. However, according to the gear pump 100, since the structure comprising the bushes 110a, 110c, the gears 115, 120 and the bushes 110b, 110d is pressed onto the rear end surface of the front cover 102 by integrally pressing them forward with a force greater than the thrust forces, the gears 115, 120 and the bushes 110a, 110b, 110c, 110d do not vibrate and the occurrence of the above-described noise and leakage problems caused by vibration is prevented.
It is noted that as a gear pump using helical gears, besides the gear pump as disclosed in the PTL 1, conventionally, there have been known a gear pump as disclosed in the Japanese Unexamined Patent Application Publication No. H2-95789 (PTL 2) and a gear pump as disclosed in the Japanese Examined Utility Model Application Publication No. S47-16424 (PTL 3).
In the gear pump disclosed in the PTL 2, the pressure of the fluid to be driven is caused to act on the shaft end surface opposite the output side of the driving gear to cause a thrust force acting on the driving shaft due to this pressure and the thrust force acting on the driving shaft due to meshing of the gears to cancel each other out.
Further, in the gear pump disclosed in the PTL 3, similarly to the gear pump disclosed in the PTL 1, a thrust force due to a pressure fluid is caused to act on each of the shaft ends of the driving gear and the driven gear to cause these thrust forces and the thrust forces acting on the driving gear and the driven gear to cancel each other out.